FIG. 1 very schematically shows an example of an interface circuit between a telephone line 1 and a user equipment 2. Line 1 is a twin-wire line and is formed of two conductors T and R (tip and ring), which are connected to the two terminals of a primary winding 3p of an isolation transformer 3. Secondary winding 3s of transformer 3 is connected to the user equipment symbolized by a block 2. The user equipment is, for example, a modem and transformer 3 then is more specifically intended for the transmission of the data exchanged between line 1 and modem 2. On the side of line 1, other control and protection equipments are generally provided. In particular, series and parallel protection circuits are generally found, as well as detection and control circuits (for example, a line take-up relay) connected on the one hand to the line and on the other hand to the modem. It should be noted that, whatever the circuit used, the electric signals must respect the isolation constraint between the line and the user equipment and thus cross, for example by means of a transformer, of an optocoupler, of capacitors or the like, a galvanic isolation barrier symbolized by a stripe-dot line IB in FIG. 1.
The detection circuits included by the interface circuit on the line side include in particular a bell detection circuit 4 (SONN). This circuit includes two input terminals 5, 6 respectively connected to conductors T and R of line 1. Circuit 4 has the function of detecting the occurrence of a bell signal on the line to enable the user equipment to pick up to receive a call, for example a fax in the case of a modem. Bell detection circuit 4 is connected to modem 2, on the user equipment side, via an isolation circuit 7 formed, in the example of FIG. 1, of an optocoupler. Optocoupler 7 is generally formed of a light-emitting diode 8, the two terminals of which are connected to two output terminals of circuit 4 and of an opto-transistor 9, the emitter and the collector of which are connected to modem 2.
On the side of user equipment 2, the secondary winding 3s of the transformer is generally connected to a so-called hybrid two wire—four wire circuit (not shown) intended for enabling the sending and receiving of a wanted signal over a same transmission line and for separating the transmitted signals from the received signals.
A disadvantage of conventional interface circuits, which translates as a functional disadvantage of modems, is that they are not able to detect the state of telephone line 1, that is, to know whether this line is busy or not. This disadvantage is particularly disturbing in the case where a same telephone line is shared between a conventional telephone equipment and a modem. In such a case, the computer exploiting the modem is incapable of knowing whether a conversation is on the line when it desires to transmit by means of the modem. Conventionally, the modem tries to perform a connection on the line, and its processing program generates an error message for the user (visible on the computer screen) to indicate that the connection has failed. This detection cannot be performed without the modem trying to take up the line. Accordingly, this results, for the user of a telephone set or for another modem occupying the line, in a pollution of the signal (audible or data pollution).